Liquefaction of methane



Nov, 7, .1950 w. H. RUPP LIQ'UEFACTION oF METHANE Filed July -8, 1947 Patented Nov. 7, 1950 LIQUEFACTION OF METHANE Walter H. Rupp, Mountainside, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application July s, 1947, serial No. 759,624 s Claims. (cl. eze-'1755) -This invention relates to the liquefaction of the lower molecular weight hydrocarbons in natural gas and in particular, relates to the liquefaction of natural gas, predominantly methane, for transportation and distribution as a domestic and industrial fuel.

In many oil elds, the natural gas hydrocarbons and the light gaseous constituents of petroleum are vented to the atmosphere and ignited as ares. This waste of valuable fuel material is due usually to the diiculty of transportation to an economically and commercially usable location. Many of the oil fields, particularly, the foreign oil elds, are located at considerable distances from processing centers and cannot be conveniently joined by pipeline to consuming centers due to intervening bodies of water. Heretofore, it has been the practice to liquefy and cool .these gases to a. temperature at which the gauge pressure is about lbs. For example, if the gas is substantially pure methane, it has been the practice to liquefy and cool the same to a temperature of about 258 F. at which temperature the absolute pressure is about 14.7 lbs. that is the gauge pressure is about 0 lbs. per sq. inch. Although higher temperatures can be used, the increase in pressure required on the large storage and shipping containers is not desirable from a safety or a cost view point. The present invention is concerned with a process by which natural gas consisting of a large proportion of methane may be liquefied in an economical manner. The liquefied gas may then be transported in liqueed form to refining and distributing centers for preparation as a, domestic and industrial fuel.

A typical gas vented to the atmosphere at many oil iields and refinery centers contains hydrocarbon constituents as follows:

In addition to the above constituents, nitrogen also occurs in amounts ranging between about 2% to 10% by volume. In some instances the concentration of nitrogen is considerably higher. Also present in most instances are hydrogen sulde, carbon dioxide and water vapor. These latter constituents are usually removed by treating the gas in conventional purication processes, which employ chemical reactants such as aliphatic amines for the removalv of hydrogen sulfide and carbon dioxide and diethylene glycol for the removal of water vapor.

The more common purification treatments for natural gas do not remove nitrogen and do not aiect the hydrocarbon constituents of the natural gas. The present invention relates to liquefaction of the natural gas, comprising predominantly methane. At the same time, the nitrogen gas, which has zero heating value and'hence is an' undesirable constituent of domestic or industrial fuels, is removed from admixture with the natural gas.

In the prior art, the liquefaction of methanev and methane-containing gases has been attained by various procedures. For example, a, conventional process is to compress gases comprising large amounts of `ymethane to' a pressure in the range from about 600 to 1000 lbs. per sq. in. The compressed gases are then cooled to a liquefaction temperature which in the case of substantially pure methane is about F. The cooled compressed methane-containing gas is then handled by conventional cascade refrigeration to cool the same to a temperature at which the liqueed gas has a gauge pressure of about 0 lbs. per sq. in. and not in excess of 5 to 10 lbs. per sq. in. methane, the temperature is about 258 F.r at which the liquefied methane has 0 lbs. gauge pressure. i

In accordance with my invention, I employ an absorption step in the liquefaction process. By the employment 0f this absorption step, I am able to reduce substantially the compression power requirements heretofore required in the liquefaction of methane and gases consisting essentially of methane. Specically, in accordance with my process, I employ an initial compression zone, a secondary absorption zone, a tertiary compression zone, a stripping zone and a methane recovery zone. In accordance with my invention, the adverse eiect of nitrogen upon liquefaction is obviated by the use of a suitable absorption liquid in the absorption zone which dissolves the methane and similar desirable components and does not dissolve nitrogen.

The process of my invention may be readily understood by reference to the drawing illustrating one embodiment of the same. This embodiment of my invention is merely illustrative and is not presented as a limitation of the invention.

Referring specifically to the drawing natural If the gas comprises essentially purev gas, which for the purpose of description is assumed to have the following composition, is introduced into initial compression zone 50 by means of line I.

Pentanes plus In compression zone 50, the gas is compressed to a pressure of about 200 lbs. per sq. in. It is to be understood that equivalency is secured for` this initial compression zone if the gas is withdrawn directly from the well at well-head pressures, which are normally in the range from about 50 to 2001bs. and considerably higher in many oil fields.

This natural gas at a pressure of about 200 lbs. per sq. in. is introduced into absorption zone I0 by means of line II and treated with a suitable absorption medium at a temperature in the range from about 125 F. to about 135 F. This temperature is maintained in the uppermost part of the absorption vessel adjacent to the point of withdrawal of the vent gases, The gases may be cooled at a point intermediate the initial compression zone and the absorption zone, or they may be introduced directly into the absorption zone and the desired temperature secured in the absorption zone by the control of the quantity and temperature of the `absorption medium, which medium is introduced into the top of absorption zone I0 by means of line I2. Temperature, pressure and absorption oil rates inl the absorption zone are controlled to secure the absorption of methane and higher boiling hydrocarbon constituents. comprising nitrogen are withdrawn from absorption zone I0 by means of line I3. The rich absorption oil containing the dissolved methane and other hydrocarbon constituents is withdrawn from absorption zone I0 by means of line I4 and compressed in compression zone I5 to a high pressure, which for the purpose of description is assumed to be about 600 lbs. per sq. in.. This oll is then passed through heat exchanging zones or equivalent means I6 andv I'I in order to raise the temperature of the same. These zones may be dispensed with, providing it is foundy more economical to supply the heat by means of the reboiler of the stripping zone.

The rich absorption oil is introduced into stripping zone wherein temperature and pressure conditions are adjusted to secure removal of methane from the absorption oil. The desired temperature is secured by withdrawing oil from bottom of zone 20 by means of line 2| and recirculating a portion of it through reboiler22, back into zone 20 by means of line 23. For the purpose of description it is assumed that the temperature maintained in the bottom of zone 20 by means of the reboiler 22 is about 400 F.

The oil substantially free of methane is withdrawn from zone 20 by means of lines 2| and 29 and introduced into auxiliary stripping zone 30. Temperatures and pressures in zone are adjusted to secure substantially complete removal of the methane from the absorption oil. This methane may be withdrawn from the system, but is preferably recycled to zone I0 by means of lines 3| and II.

The absorption oil substantially completely free Undissolved Vent gases of methane is withdrawn from zone 30 by means of line I2. This absorption oil is passed through cooling zone 60 and then reintroduced into the top of absorber I0 by means o1' line I2 as heretofore described. In order to maintain the desired temperature in the bottom of auxiliary stripper 30, the absorption oil withdrawn from zone 30 is passed through reboi1er33 and the necessary quantity recycled to zone 30 by means of line 3l.

'I'he methane and hydrocarbon constituents are removed from the absorption oil in zone 20 by means of line 24. This stream is passed through exchanger 25 in which the stream is cooled to secure liquefaction of the same at the pressure existing in stripper 20. When the stream comprises essentially methane, it is cooled to about 130 F. at 600 lbs. pressure. The liquid stream is passed through line 21 to cascade separators and suitable pressure reducing equipment 28. These devices reduce the pressure in stages to about 0 lbs. per` sq. in. gauge, the storage pressure. Gases evolved from zone 28 are removed by means of lines 31 and 38 and preferably recycled to the absorbers. Liqueed methane is withdrawn from zone 28 by means of line 39 and passed to suitable storage equipment, which for 'the purpose of illustration is shown to be underground storage tanks 40.

The process of my invention comprises essentially subjecting a gas comprising methane to absorption at a pressure in the range from 50 to 500 lbs. per sq. in. In general, my invention is adapted to the treatmentv of natural gas, the` composition of which comprises in excess of '75% by Volume of methane. It is particularly adapted when the composition of the natural gas is in excess of by volume of methane, excluding the quantity of nitrogen, hydrogen sulfide and carbon dioxide present.

My invention is particularly adapted to the treatment of naturalgases which are evolved at well-head pressures in the range from 50 to 200 lbs. per sq. in. These gasesare introduced into an absorption zone at these pressures. It is to be understood, however, that the 'invention covers the actual compression of natural gases to the pressures in the range from 50 to 500 lbs. per sq. in., preferably under pressures in the range from 50 to 200 lbs. per sq. in. Temperature and pressure conditions in the absorption zone are adjusted to secure substantially complete absorption of methane and higher boiling hydrocarbon constituents from the gases to the exclusion of the nitrogen present. In general,l the temperatures maintained in the absorption zone are in the range from about to 150 F., preferably in the range from F. to 135 F. It is to be understood that temperature and pressure conditions may be varied,. depending upon the composition of the feed gases, the type of absorption oil used and the quantity of absorption oil used.

The absorption oil may be any satisfactory mineral oil. In general, it is preferred that it be a liquid hydrocarbon, preferably, -a saturated petroleum hydrocarbon comprising essentially C5 constituents. Any substantially saturated hydrocarbon fraction boiling in the range from about plus 10 F. to plus 170 F. may be used as the solvent oil. However, as pointed out heretofore, a preferred solvent oil comprises a saturated hydrocarbon fraction, the constituents of which are essentially in the range from C4 to Cs parainic hydrocarbons.

In general, the temperature maintained in the top of the absorption zone is in the range from 125 F. to 135 F., while the temperature of the feed oil withdrawn from the bottom of the absorption zone is in the range from about 110 F. to 125 F. This absorption oil is then passed to a compression zone wherein the pressure is increased on the same to a pressure in the range from about 500 to 600 lbs. per sq. in. The compressed oil is then passed to a stripping zone, the conditions of which are adjusted to secure substantially complete removal of the methane from the absorption oil. In general, the temperature in the bottom of the stripping zone is maintained in the range from about 300 F. to 450 F. The methane is removed overhead from the stripping zone and cooled to a temperature adapted to liquefy the same. This temperature will vary depending upon the pressure, but is generally in the range from about 120 F. to 150 F.

'Ihe methane and methane-containing liquid is then further cooled to a pressure, preferably about 0 lbs. gauge and not in excess of about 10 lbs. gauge. If the liquid comprises essentially pure methane, this temperature is about 250 F. In general, the liqueed methane and methane-containing liquid is subsequently cooled to a temperature in the range from about 250 F. to 270 F.

The process of my invention is not to be limited by any theory as to mode of operation, but only in and by the following claims:

What is claimed is:

l. An improved process for liquefying natural gas predominantly methane in character which comprises contacting said natural gas under a pressure from about 50 to 500 lbs. per sq. in. with a hydrocarbon liquid absorption oil in an absorption zone under conditions to absorb substantial- 1y all the methane and higher boiling hydrocarbon constituents,I removing the absorption oil containing the absorbed constituents from said absorption zone and compressing the same to a higher pressure in an intermediate compression zone, withdrawing the compressed absorption oil from said intermediate compression zone at said higher pressure and introducing the same into a separation zone, separating methane from said absorption oil at said higher pressure in said separation zone, removing the methane overhead from said separation zone and cooling the methane to liquefy the same at said higher pressure and then further cooling said methane to secure a pressure in the range below about lbs. per sq. in. gauge.

2. Process as dened by claim 1 wherein said pressure in said absorption zone is in the range from about 50 to 200 lbs. per sq. in.

3. Process as defined by claim 2 wherein said higher pressure is in the range from about 500 to 600 lbs. persq. in.

4. Process as defined by claim 3 wherein said methane removed from said absorption oil is cooled to a temperature in the range from about 125 F. to 135 F. at said higher pressures.

5. An improved process for liquefying natural gas containing a large percentage of methane and free from hydrogen sulfide, carbon dioxide and water, which comprises compressing said natural gas in an initial compression zone toa. pressure in the range from about 50 to 500 lbs. per sq. in., passing said compressed gas to an absorption zone and treating the same in said absorption zone with an absorption oil adapted to dissolve methane and higher boiling hydrocarbon constituents in said absorption oil, removing said absorption oil containing said methane and higher boiling hydrocarbon constituents and further compressing the same to a higher pressure. subjecting said oil at said higher pressure to elevated temperatures adapted to separate the methane and said higher boiling hydrocarbon constituents from the absorption oil, cooling said separated methane and higher boiling hydrocarbon constituents to a temperature adapted to liquefy the same at said higher pressure, followed by further cooling said methane and said hydrocarbon constituents to secure a gauge pressure less than about 10 lbs. per sq. in. gauge.

6. Process as defined by claim 5 wherein said pressure in said initial compression zone is in the range from about 50 to 200 lbs. per sq. in.

7. Process as defined by claim 5 wherein said higher pressure is in the range from about 500 to 600 lbs. per sq. in.

8. Process as defined by claim 5 wherein said methane and said higher boiling hydrocarbon constituents separated from said absorption oil is cooled to a temperature in the range from about to 135 F. and then further cooled to a temperature of about 250 F. to 270 F.

WALTER H. RUPP.

REFERENCES CITED The following references are of record in the le of this patent:v

UNITED STATES 'PA'I'ENTS Number Name Date 668,197 Le Sueur Feb. 19. 1901 2,264,878 Hatch Dec. 2, 1941 2,374,104 Kirkbride Apr. 17, 1945 

1. AN IMPROVED PROCESS FOR LIQUEFYING NATURAL GAS PREDOMINANTLY METHANE IN CHARACTER WHICH COMPRISES CONTACTING SAID NATURAL GAS UNDER A PRESSURE FROM ABOUT 50 TO 500 IBS. PER SQ. IN WITH A HYDROCARBON LIQUID ABSORPTION OIL IN AN ABSORPTION ZONE UNDER CONDITIONS TO ABSORB SUBSTANTIALLY ALL THE METHANE AND HIGHER BOILING HYDROCARBON CONSTITUENTS, REMOVING THE ABSORPTION OIL CONTAINING THE ABSORBED CONSTITUENTS FROM SAID ABSORPTION ZONE AND COMPRESSING THE SAME TO A HIGHER PRESSURE IN AN INTERMEDIATE COMPRESSION ZONE, WITHDRAWING THE COMPRESSED ABSORPTION OIL FROM SAID INTERMEDIATE COMPRESSION ZONE AT SAID HIGHER PRESSURE AN INTRODUCING THE SAME INTO A 